Polyolefin macroporous membranes have been widely used not only as battery separators used for lithium secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, polymer batteries, etc. but also as electrolytic capacitor separators, various filters, such as reverse osmosis filtration membrane, ultrafiltration membrane and microfiltration membrane, moisture-permeable waterproof clothes, medical materials, etc.
When a polyolefin microporous membrane is used as a battery separator, particularly as a lithium ion battery separator, its performance deeply relates to battery properties, battery productivity and battery safety. On that account, for the polyolefin microporous membrane, excellent mechanical properties, heat resistance, permeability, dimensional stability, shutdown properties, meltdown properties, etc. are required. For example, if a polyolefin microporous membrane having low mechanical strength is used as a battery separator, voltage of the battery is sometimes lowered because of short circuit of an electrode.
A microporous membrane made of polyethylene alone generally has low mechanical strength, and in order to improve mechanical strength, a microporous membrane comprising ultrahigh-molecular weight polyethylene has been proposed. For example, a separator made of a composition containing ultrahigh-molecular weight polyolefin as an essential component has been proposed in a patent literature 1, a patent literature 2 or the like.
As for the properties of separators, however, not only mechanical strength and permeability but also safety has been strictly regarded as important recently. In particular, an electrode of a lithium ion battery repeatedly undergoes expansion and shrinkage accompanying electric charge and discharge. In this case, shutdown properties to rapidly shut down the battery circuit when high electric current flows because of external short circuit or the like are required. As a separator of a lithium ion battery, a polyethylene microporous membrane produced by a stretching pore-opening method or a phase separation method is practically used at present, and the reason is that the membrane is melted at a relatively low temperature by heat generated by the short-circuit current to close the micropores, whereby the battery circuit can be shut down, and the temperature rise after closure of the micropores can be inhibited.
For the microporous membrane of the lithium ion battery, however, not only such a function to close micropores at a relatively low temperature but also an ability to retain the shape when the temperature is raised to a high temperature is important, and if the shape is not retained, a dangerous state of direct contact of electrodes, that is, meltdown may be brought about. This meltdown temperature of the battery separator made of polyethylene cannot be said to be satisfactory, because the separator has a low melting point. In order to improve the meltdown properties, a separator composed of polyethylene and polypropylene has been proposed in a patent literature 3. As for the ratio between the polyethylene and the polypropylene, this literature refers to a polyethylene content of not more than 20 parts by mass. However, the meltdown temperature of this microporous membrane is not particularly high, and it cannot be said that the heat resistance of the polyolefin microporous membrane is satisfactory. In a patent literature 4, a microporous membrane composed of polyethylene and a non-polyethylene-based thermoplastic resin has been proposed. As the polyethylene, ultrahigh-molecular weight polyethylene has been proposed, but the substance used in this literature is a polyethylene composition (mixture of ultrahigh-molecular weight polyethylene and high-density polyethylene) containing high-density polyethylene as a main component, and its molecular weight is not high. On that account, it cannot be said that the microporous membrane obtained from the composition has strength enough to stand its use, and besides, the heat resistance is not sufficient either. In a patent literature 5, a polyolefin microporous membrane made of a mixture of polyethylene and polymethylpentene has been proposed. In this literature, it is disclosed that in order to produce a polyolefin microporous membrane, a mixture (polyolefin resin) of high-density polyethylene and polymethylpentene is melt kneaded and then stretched at a specific preset temperature. The resulting microporous membrane, however, is insufficient in points of strength and heat resistance, similarly to the patent literature 4. In this literature, it is further described that in the case where ultrahigh-molecular weight polyethylene is used, a fraction having a molecular weight of not more than 1,000,000 must be not less than 80% by weight based on the whole system in order to obtain a homogeneous composition. In this literature, however, it is not disclosed at all that the performance of the microporous membrane can be enhanced by increasing the molecular weight of the polyolefin resin.
In view of such prior art as mentioned above, a polyolefin resin composition which has particularly excellent heat resistance in addition to excellent mechanical properties, permeability, dimensional stability and shutdown properties and is suitable particularly for a microporous membrane has been desired in the market.